Currently available combined cycle systems of the assignee of this invention include single and multi-shaft configurations. Single shaft configurations may include one gas turbine, one steam turbine, one generator and one heat recovery steam generator (HRSG). The gas turbine and steam turbine are coupled to the single generator in a tandem arrangement on a single shaft. Multi-shaft systems, on the other hand, may have one or more gas turbine-generators and HRSG's that supply steam through a common steam header to a single steam turbine generator. In either case, steam is generated in one or more unfired HRSG's for delivery to the condensing steam turbine.
Conventional practice for reheat steam cycles utilizing reheat is generally to configure the steam system with one steam turbine to one HRSG. In a single gas turbine/steam turbine/HRSG system, condensate from the condenser is pumped directly to the HRSG where it is reheated by the gas turbine exhaust gases, and is thereafter returned to the steam turbine.
To start a cold steam turbine system in a combined cycle plant normally requires the gas turbine to be at a low load so that the steam to the steam turbine is at a low temperature compatible to the cold steam turbine metal. Failure to do so shortens the steam turbine life due to low cycle fatigue of parts, distorted casing and shaft, rubbed seals and blades, etc. However, running the gas turbine at low load not only reduces power generated from the gas turbine but also wastes fuel. Alternatively, where the gas turbine load is not reduced, the high temperature steam must be passed through an attemporating station to reduce the steam to the appropriate temperature.